jemalloc doesn't work on DragonFly

jemalloc is a malloc(3) implementation from FreeBSD and for some reason
increasingly popular in various software pieces dealing ECMAscript and related
stuff. This means browsers (Firefox, Chrome), but also Gnash for example.
Unfortunately jemalloc doesn't work with DragonFly. Matt explained why some
time ago:

Looking into it, my port just uses _SPINLOCK for malloc_mutexes; to be able to
do so, we'd need to include /usr/src/lib/libc/include/libc_private.h ; it'd be a
bit of a shame if Firefox's code had to do the same.